Twins without fertility boosters!

Note, I am trying to figure out if the twin thing is based on your emotional level or the sims age level, I do know when they are earlier in the young adult stage they have a higher chance of getting twins, I think it has something to do with the couples experience when trying for a baby, not really tried it. However if your close to adult the twins chance becomes reduced, same goes for Adult, earlier in adult age your sim has most likely your last chance to have twins, but as your sim ages your sim is not always successful the closer to elderly age you get, also it also depends on the age difference between the male sim and female sim if both are really young your get twins, but if both are not you may get them or you may not get many at all, and if your sim was born a twin, your not likely to get twins from that group, but the kids of twins may have a higher chance of getting twins.

Edit: Some of this stuff is what I know about and what I think from the information available, it is really heard for me to test it out because at the moment my game has no need for kids at the moment.